In addition to the conventional vehicle suspension systems formed of leaf or helical springs having a constant stiffness at all loading conditions, pneumatic and hydropneumatic suspensions are known in which the stiffness varies according to an exponential law determined by the adiabatic transformation of the compressed gas forming the elastic suspension element.
Such pneumatic and hydropneumatic suspensions are more compact and reliable than those having a mechanical elastic element, but as the compression and expansion curve of the gas is not linear their performance is not so good over the entire range of applied loads from zero to maximum loading.
As the gradient of this curve increases with increased yielding of the suspension, it is evident that an approximate linearity of response of the suspension can only be maintained when the difference between maximum and minimum loading is relatively small such as in the case of the front axle of a truck. However, when the difference between maximum and minimum loading is very high such as in the case of the truck body supporting axle, it is impossible to obtain a satisfactory performance both at maximum and minimum loading because when the suspension is dimensioned for adequate behaviour at maximum loading, it will yield too much without load and, on the other hand, when the suspension is dimensioned to behave properly without load, it will be too rigid at full load.
The compromise solutions hitherto adopted are unsatisfactory not only because the poor behaviour of the suspension will reduce the comfort of the passenger or driver, but above all because it will be detrimental to the stability and safety of the vehicle as it is running on the road.
To achieve normal stiffness of the suspension in all conditions of loading, the present applicant has already proposed and used an outer chamber located outwardly of the suspension and communicating with the compressed gas inside the suspension, this outer chamber containing gas at higher pressure which acts to change the behaviour of the suspension at high loading. This solution has permitted to extend the range of use of the suspension but at the expense of increased overall dimensions and higher costs of production.